Device for obtaining and switching high voltages applied to x-ray tube electrodes

ABSTRACT

Disclosed is a device that can be used to obtain voltages for the biasing of the electrodes used to deflect the electron beam emitted by a cathode of an X-ray tube. Four DC voltages ±V 1  and ±V 2  are generated and applied to a mixing circuit in which they are added by using diodes to floating reference pulse voltages ±V 3  so as to obtain voltages VS 1 , VS 2 , VS&#39; 1 , VS&#39; 2  for the charging of capacitors such that: 
     
         VS.sub.1 =V.sub.3 -V.sub.1 and VS.sub.2 =-V.sub.3 +V.sub.2 
    
     
         VS&#39;.sub.1 =-VS.sub.3 +V.sub.2 and VS&#39;.sub.2 =V.sub.3 -V.sub.1

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to radiology instruments and, moreparticularly, it relates to devices for biasing electrodes of X-raytubes, with high voltages and for switching the same speedily.

An X-ray tube comprises, in a vacuum chamber, a cathode constituted by aheated filament that emits electrons and a focusing device baked on tothe filament that focuses the emitted electrons on an anode at apositive potential with respect to the cathode. The point of impact ofthe electron beam on the anode constitutes the X-radiation source.

To shift the X-ray beam angularly, it is generally proposed to shift thepoint of impact of the electron beam on the anode by using a deflectingdevice. This deflecting device is usually constituted by magnetic orelectrostatic lenses which are placed on the path of the beam or in thevicinity of this path between the cathode and the anode. The use ofthese lenses necessitates a non-negligible level of energy owing to thesubstantial kinetic energy of the electrons of the beam. Thissubstantial kinetic energy is due to their high speed as a result ofhigh potentials difference of more than 100 kilovolts, between thecathode and the anode.

2. Description of the Prior Art

In French patent No. 2 538 948, there is disclosed an X-ray tube withscanning, in which the focusing device has at least two metal parts thatare electrically insulated from one another and from the filament so asto enable them to be biased or polarized independently with respect tothe filament, and so as to thus obtain a deflection of the electronbeam.

FIG. 1 gives a schematic view of an X-ray tube of the type described inthe above-mentioned patent. It comprises, in a vacuum chamberrepresented by the dashed rectangle 11, a filament 12, a focusing device13 backed on to the filament 12 and an anode 14. The filament 12 and thefocusing device 13 constitute a cathode C. The focusing device 13 isconstituted by a first metal part 15 and a second metal part 16 that areelectrically insulated from each other by an insulating partition wall17 fixedly joined to an insulating base 18. The metal parts 15 and 16are placed symmetrically on either side of the filament 12 with respectto a plane of symmetry perpendicular to the plane of FIG. 1. This planeof symmetry contains the axis of the filament 12 perpendicular to theplane of FIG. 1. This plane of symmetry contains the axis of thefilament 12 perpendicular to the plane of FIG. 1 and is perpendicular tothe base 18. The intersection of this plane of symmetry with the planeof FIG. 1 defines the axis 19 of the electron beam.

The voltages that are applied to the metal parts 15 and 16 are chosen soas to focus the electrons on a determined surface of the anode 14. Inmodifying these voltages, it is possible to shift the point of impact ofthe electron beam on the anode, but the characteristics of the impactsurface are also modified.

Thus, to obtain relatively major deflections of the electron beamwithout modifying the characteristics of the impact surface, a knownmethod uses two additional metal electrodes W₁ and W₂ which are borne bythe focusing piece 13 at the end of the last stepped portion of eachmetal part 15 and 16.

These electrodes W₁ and W₂ are respectively insulated from the metalparts 15 and 16 by an insulating portion 20 or 21, made of alumina forexample.

When no voltage VS₁ or VS₂ is applied between the filament 12 and,respectively, the electrode W₁ and the electrode W₂, an electron beam Fis emitted along the axis 19.

When equal voltages are applied to the electrodes W₁ and W₂, the cathodeC emits an electron beam F along the axis 19, the concentration of whichis obtained by the geometry of the cathode C.

To obtain a deflection of the electron beam, namely to give this beam amean direction that is different from the axis 19, it is sufficient tointroduce a dissymmetry into the electric field created around theelectron beam by giving different values to the voltages VS₁ and VS₂applied to the metal electrodes W₁ and W₂. Thus, a beam F' with an axis19' is obtained for a positive voltage VS₁ and a negative voltage VS₂.On the contrary, a beam F" with an axis 19" is obtained for a negativevoltage VS₁ and a positive voltage VS₂.

To obtain a deflection of the beam of the order of one millimeter toseveral millimeters in the case of a filament-anode distance of twentymillimeters, it is necessary to apply voltages VS₁ and VS₂ of the orderof 2,000 to 3,000 volts.

More precisely, as can be seen in the graphs of FIGS. 2-a and 2-b, equaland opposite voltages of +V₁ are applied to the electrodes W₁ and W₂ toobtain a (1 certain position of the focal spot, and equal and oppositevoltages of +V₂ are applied to obtain another position of the focalspot. It is therefore necessary to switch voltages over from +V₁ to -V₂and then again to +V₁ on the electrode W₁ and from -V₁ to +V₂ and thenagain to -V₁ on the electrode W₂.

FIG. 3 shows a block diagram of a device for switching the voltages ±V₁and ±V₂. It comprises four voltage generators G₁, G₂, G₃ and G₄ whichrespectively provide the voltages +V₁, -V₁, -V₂ and +V₂, these voltagesbeing applied to the electrodes W₁ and W₂ by means of switches I₁, I₂,I₃ and I₄, the opening and closing of these switches being activatedrespectively by signals P₁, P₂, P₃ and P₄ given by a control circuit P.By simultaneously closing the switches I₁ and I₂, with I₃ and I₄ beingopen, a voltage +V₁ is applied to W₁ and a voltage -V₁ is applied to W₂.Similarly, by simultaneously closing I₃ and I₄, with I₁ and I₂ beingopen, a voltage -V₂ is applied to W₁ and a voltage +V₂ is applied to W₂.

There are many known electronic circuits that can be used to perform thefunctions described in relation to the block diagram of FIG. 3. For thisswitching operation, increasing use is being made of metal-oxide typefield-effect transistors or MOSFETs. However, these transistorsgenerally cannot withstand voltages of more than some hundreds of voltswhich means that several of them (for example seven of them) have to beplaced in series to enable the switching of a voltage of severalthousands of volts. Furthermore, it is necessary to have a power controlcircuit for all the transistors of the switch for the application of thesignals P₁ to P₄, which leads to design appropriate power circuit.

Besides, it happens that the X-ray tube gets short-circuited between thecathode and the anode, and the result thereof is that the transistors ofthe switch are subjected to electromagnetic disturbances against whichthey should be shielded, for example by using clamping circuits. Aswitching device such as this therefore leads to the use of many costlyand bulky components.

An object of the present invention, therefore, is to design a device forswitching high bias voltages, that is simple, inexpensive and does notuse switching transistors.

At last, switching transistors as well as their control and protectioncircuits are borne to a potential of -75 kilovolts in relation to theground, which makes it necessary to place them in an insulating highvoltage unit, namely in an insulating chamber containing an insulatingfluid, said high voltage unit also providing the high voltages appliedto the cathode and the anode.

Hence, another object of the present invention is to design a device forswitching high biasing voltages that switches said voltages with respectto the ground and not with respect to the cathode potential of -75kilovolts, thus enabling it to be placed outside the high voltage unitwhich produces the supply voltages of the cathode and of the anode.

SUMMARY OF THE INVENTION

The invention relates to a device used to obtain and to switch X-raytube electrode biasing voltages VS₁, VS₂ or VS'₁, VS'₂, said devicecomprising:

first means for generating a first pair of adjustable DC voltages +V₁,-V₁ with equal and opposite amplitudes,

second means for generating a second pair of adjustable DC voltages +V₂,-V₂ with equal and opposite amplitudes,

third means for generating a pair of pulse voltages +V₃, -V₃ with equaland opposite amplitudes, and

fourth means connected to the first, second and third means, forcombining the pair of pulse voltages +V₃, -V₃, at determined instants,with one of the voltages of the first and second pairs of DC voltages soas to charge capacitors and obtain DC voltages

    VS.sub.1 =V.sub.3 -V.sub.1 and VS.sub.2 =-V.sub.3 +V.sub.2

during a certain period of time, and then DC voltages

    VS'.sub.1 =-V.sub.3 +V.sub.2 and VS'.sub.2 =+V.sub.3 -V.sub.1

during another period of time.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the invention will appear fromthe following description of an embodiment, said description being madewith reference to the appended drawings, of which:

FIG. 1 is a schematic sectional view of a cathode and a anode of anX-ray tube that has electrodes for deflecting the electron beam;

FIGS. 2-a and 2-b are timing diagrams of the voltages that are appliedto the deflection electrodes;

FIG. 3 is a diagram illustrating the principle of a bias voltageswitching device;

FIG. 4 is a diagram of the device used for obtaining and switching overbias voltages according to the invention;

FIGS. 5-a to 5-d are timing diagrams of signals, to provide for anunderstanding of the way in which the voltages are obtained;

FIG. 6 is a diagram of a control circuit 33 (or 43) of the convertercircuit 34 (or 44);

FIG. 7 is a graph showing the curve of calibration of the circuit 30 orof the circuit 40 of the device of FIG. 4;

FIGS. 8-a to 8-f are graphs enabling an understanding of the operationof the circuits 30 and 40 of the device of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

The device, according to the invention, for obtaining and switchingvoltages comprises (FIG. 4) a first circuit 30 for obtaining thevoltages +V₁ and -V₁, a second circuit 40 for obtaining the voltages +V₂and -V₂ and a device 29, for switching said voltages ±V₁ and ±V₂ towhich these latter voltages are applied.

The first and second circuits 30 and 40 are identical, and each of themhas the following circuits which have been described in the Frenchpatent application No. 90 10348 filed on 14 August 1990 and entitled"Device for obtaining an adjustable DC voltage".

Firstly there is a microprocessor 31 (or 41) to which the userindicates, at its input terminal 50 (or 51), for example by means of akeyboard, the voltage ±V₁ (or +V₂) to be obtained. The microprocessor 31(or 41) gives a digital code N₁ (or N₂) which means a frequency F₁ (orF₂). This digital code N₁ (or N₂) is applied to a programmable counter32 (or 42) which gives pulses of variable frequency F₁ (or F₂) accordingto the value of N₁ (or N₂). These pulses are applied to a controlcircuit 33 (or 43) which produces pulses for controlling a hyporesonanttype converter circuit 34 (or 44). The output signals of the convertercircuit 34 (or 35) are applied to a primary winding 35p (or 45p) of apulse type transformer 35 (or 45), the secondary winding 35s or (45s) ofwhich is connected to a rectifying and filtering circuit 36 (or 46)having three output terminals 37, 38 and 39 (or 47, 48 and 49). Theoutput terminal 37 (or 47) is at the potential +V₁ (or +V₂) with respectto the terminal 39 (or 49). The output terminal 38 (or 48) is at thepotential -V₁ (or -V₂) with respect to the terminal 39 (or 49); theoutput terminal 39 (or 49) is connected to a terminal 58 of the filament12 (FIG. 1) and is therefore at the potential of this filament.

These different output terminals 37, 38 and 39 (or 47, 48 and 49) areconnected to the switching device 29. The switching circuit 29 comprisesa device 52 for obtaining a DC voltage E that is regulated andadjustable, a circuit 53 for switching the voltage E, a pulse typetransformer 54 and a mixer circuit 55 for mixing the voltages +V₁, +V₂with the voltages given by the transformer 54 so as to obtain thevoltages applied to the electrodes W₁ and W₂ at output terminals 56, 57and 58. These output terminals 56, 57 and 58 are respectively connectedto the electrode W₁, the electrode W₂ and the emissive filament of thecathode (FIG. 1).

A detailed description shall now be given of the different circuits thathave been presented here above in a functional way.

The microprocessor 31 (or 41) performs the function:

    N.sub.1 =f|V.sub.1 |(or N.sub.2 =f|V.sub.2 |)

i.e., for each value of the voltage |V₁ | (or |V₂ |), it gives a digitalcode, for example a code with eight digits. This code, when applied tothe counter 32 (or 42), causes said counter to provide frequency F₁ (orF₂) pulses. These frequency F₁ (or F₂) pulses are designed to controlthe switches of the converter circuit 34 (or 44) alternately by means ofthe circuit 33 (or 43), so as to create current pulses. The rectifyingand filtering of these current pulses in the circuit 36 (or 46) lead tothe desired voltages +V₁ (or +V₂) between the terminals 39, 37 and 39,38 (or 49, 47 or 49, 48).

In other words, the microprocessor 31 and the counter 32 perform thefunction F₁ =f'|V₁ | (or F₂ =f'|V₂ |). This function is obtained bycalibration and its shape is given by the curve 81 of FIG. 7. This curve81 takes account of the linearity defects of the system while the curve80 is a theoretical curve.

The control circuit 33 (or 43) has (FIG. 6) a first logic AND circuit 82that comprises two input terminals, to one of which are applied thepulses of variable frequency F₁ (or F₂) given by the circuit 32 (or 42)while the other input terminal is connected to a [first delay circuit83, the delay of which is θ₁. The output terminal of the AND circuit 82is connected, firstly, to a bistable circuit 85 and, secondly, to thefirst delay circuit 83 as well as to a second delay circuit 84, thedelay of which is θ₂.

The output terminal corresponding to the state 1 of the bistable circuit85 is connected to one of the two input terminals of a second logic ANDcircuit 86 while the output terminal corresponding to the state 0 isconnected to one of the two input terminals of a third logic AND circuit87. The second input terminal of the AND circuits 86 and 87 is connectedto the output terminal of the second delay circuit 84. The outputterminals of the AND circuits 86 and 87 respectively bear the references33a and 33b.

The converter circuit 34 (or 44) has at least two switches T₃, T₄ (orT₅, T₆), made with field-effect transistors according to metal-oxidesemiconductor technology (i.e. these transistors are MOSFETs). Byconstruction, each of these transistors T₃, T₄ (or T₅, T₆) has, inparallel, a diode D₁₄ (or D₁₆) for the transistor T₄ (or T₆) and a diodeD₁₃ (or D₁₅) for the transistor T₃ (or T₅). The anode of each of thesediodes is connected to the source of the associated transistor and thecathode of each of said diodes is connected to the drain of theassociated transistor. The gate of the transistor T₄ (or T₆) isconnected to the output 33a (or 43a) of the control circuit 33 while thegate of the transistor T₃ or T₅ is connected to the output 33b (or 43b)of the control circuit 33 (or 43).

The converter circuit also includes a resonant circuit comprisingcapacitors C₅ and C₆ (or C₇ and C₈) and a coil L₁ (or L₂) The capacitorsC₅ and C₆ are series-connected between the drain of the transistor T₄and the source of the transistor T₃, while the coil L₁ (or L₂) isconnected in the primary circuit 35p (or 45p) of the transformer 35 (or45). This coil L₁ (or L₂) is connected, on one side, directly to thesource of the transistor T₄ (or T₆) and, on the other side, to thecommon point of the capacitors C₅ and C₆ (or C₇, C₈), by means of theprimary winding 35p (or 45p) of the transformer 35 (or 45). In a knownvariation, the converter circuit may have only one capacitor instead oftwo capacitors C₅ and C₆ (or C₇, C₈). This single capacitor would beconnected, for example, to the negative terminal of the supply circuit52.

The rectifying and filtering circuit 36 (or 46) is of a standard type,and comprises rectifier diodes D₅ and D₆ (or D₇, D₈) and filteringcapacitors C₁ and C₂ (or C₃, C₄) which are connected to each other in aknown way. The output impedance of the circuit 36 (or 46) is constitutedby two resistors of equal values R₁ and R₂ (or R₃ and R₄), the commonpoint of which constitutes the output terminal 39 (or 49) which isconnected to the common point of the capacitors C₁ and C₂ (or C₃ andC₄). The voltage +V₁ (or +V₂) is then obtained between the terminals 39and 37 (or 49 and 47) and the voltage -V₁ (or -V₂) is obtained betweenthe, terminals 39 and 38 (or 49 and 48).

The operation of the circuit 30 alone shall now be explained with thehelp of FIGS. 4, 6, 7 and 8, the operation of the circuit 40 beingidentical. To a bias voltage +V₁, there corresponds a digital code N₁.This digital code N₁, when applied to the counter 32, leads this counterto provide pulses 72 and 72' (FIG. 8-a) at the frequency F₁ according tothe correspondence given by the curve 81 of FIG. 7. These pulses have,for example, a frequency of 30 kilohertz to obtain |V₁ |=3,000 volts anda duration of about one microsecond. If it is assumed that the delaycircuit 83 gives an opening signal 73, the pulse 72 activates thechanging of the state of the bistable circuit 85 which switches, forexample, to the state 1. The pulse 72 activates the delay circuit 83 toend the opening signal 73 (FIG. 8-c) so that the AND circuit 82 closesduring a period of time θ₁. The pulse 72 also activates the delaycircuit 84 to make it provide a signal T'₄ with a duration θ₂ (FIG. 8-b)that turns the AND circuits 86 and 87 on. Only the AND circuit 86, whichreceives the state 1 signal from the bistable circuit 85, gives a signalT'₄ that makes the transistor T₄ conductive at the instant t_(o) (FIG.8-d).

This signal T'₄ makes the transistor T₄ conductive and keeps it in thisstate, and a current i₁ (FIG. 8-d), called a positive current, flows inthe transistor T₄, the coil L₁, the primary winding 35p of thetransformer 35, the capacitors C₅ and C₆ (in fact i₁ /2 in eachcapacitor) and the supply circuit 52.

This current i₁ leads to a square-wave voltage V (FIG. 8-e) at theterminals of the primary winding 35p, and the result thereof is acurrent I(t) (FIG. 8-f) in the secondary winding 35s of the transformer35. This current has a shape identical to that of the current i₁ flowingin the primary winding.

The current i₁ charges the capacitor C₅ and discharges the capacitor C₆and their charging voltage counters the flow of the current i₁ so thatthis current i₁ gets cancelled out at the instant t₁, i.e. before theend of the signal T'₄. The capacitor C₅ then gets discharged while thecapacitor C₆ gets charged and a current i₂ (FIG. 8-d), called a negativecurrent, flows in the capacitors C₅ and C₆ (in fact i₂ /2 in eachcapacitor), the primary winding 35p, the coil L₁, the diode D₁₄ and thesupply circuit 52.

This negative current leads to a square-wave negative voltage (FIG. 8-e)at the terminals of the primary winding 35p and, consequently, to anegative current I(t) (FIG. 8-f) in the secondary winding 35s. When thecurrent i₂ gets cancelled out, the pulse is ended.

Before the instant t₂, the signal T'₄ comes to an end by the effect ofthe delay circuit 84 introducing a delay θ₂ so that the AND circuits 86and 87 are off.

After the instant t₂, and more precisely after a delay θ₁ measured fromthe end of the signal 73 (FIG. 8-c), the delay circuit 83 gives a signal73' that turns the AND circuit 82 on.

After a variable period of time defined by the frequency F₁, a pulse 72'is provided by the counter 32, and its leading edge activates the changein the state of the bistable circuit 85, which switches to the state 0,as well as the zero-setting of the delay circuits 83 and 84.

This zero-setting operation has the effect of ending the signal 73' andof giving the signal T'3 which opens the AND circuits 86 and 87. Sincethe bistable circuit 85 is in the state 0, only the AND circuit 87 givesan output signal at the terminal 33b and a pulse is applied to thecontrol electrode of the transistor T₃ at the instant t'_(o) to make itconductive. A current i'₁, called a negative current, then flows in thetransistor T₃, the circuit 52, the capacitors C₅ and C₆ (in fact i'₁ /2in each capacitor), the primary winding 35p of the transformer 35 andthe coil L₁. This negative current leads to a square-wave negativevoltage V (FIG. 8-e) at the terminals of the primary winding 35p, andthe result thereof is a negative current I(t) (FIG. 8-f) in thesecondary winding 35s of the transformer 35. This current has a shapeidentical to that of the current i'₁ flowing in the primary winding.

The negative current i'₁ charges the capacitor C₆ and discharges thecapacitor C₆ and their charging voltage counters the flow of the currenti'₁ so that this current i'₁ gets cancelled out at the instant t'₁. Thecapacitor C₆ then gets discharged while the capacitor C₆ gets chargedand a positive current i'₂ flows in the capacitors C₅ and C₆ (in facti'₂ /2 in each capacitor), the primary winding 35p, the coil L₁, thediode D₁₃ and the supply circuit 52. This positive current leads to asquare-wave positive voltage (FIG. 8-e) at the terminals of the primarywinding 35p and, consequently, to a positive current I(t) (FIG. 8-f) inthe secondary winding 35s. When the current i'₂ gets cancelled out, thepulse is ended.

The pulses thus created by the converter circuit 34 are applied to thetransformer 35 and are rectified and filtered in the circuit 36 so that,at the terminals of the each resistor R₁ and R₂, there appears a voltageV₁ corresponding to the frequency F₁ determined by calibration.

This relationship between the frequency F₁ and the voltage V₁ resultsfrom the fact that the electrical charge contained in each pulse (FIGS.8-d and 8-f) is always the same whatever may be the point of operation,provided that the frequency F₁ is lower than the frequency of theresonant circuit of the converter circuit. This means that the ripplecircuit is of the pulse hyporesonant type.

As a matter of fact, the electrical charge Q of a pulse (FIG. 8-d) isgiven by: ##EQU1## with

E: the supply voltage

V: the voltage at the terminals of the primary winding 35p ##EQU2##

From the above, it can be deduced Q=2 CE, i.e. a constant if E and C areconstant, which is the case as the supply circuit 52 gives a regulatedvoltage and the capacitance C is fixed in manufacture.

Now the current I₄ that flows in the load resistor R₁ is given by:

    I.sub.r =Q×F.sub.1

so that the voltage V₁ =R₁ =R₁ ×Q×F₁, which means that V₁ isproportional to F₁ if R₁ and Q are constants. This corresponds to thedashed curve 80 of FIG. 7. However, in practice, the phenomenon is notperfectly linear and the real curve is the one referenced 81. If thedevice according to the invention is to work according to the curve 81,it is necessary to carry out a calibration in using at least two pointsof operation, for example those defined by A and B on the curve 81.

In the switching circuit 29, the circuit 52 for obtaining the DC voltageE comprises a first rectifying and filtering circuit 59 which issupplied by the AC mains system between the terminals 63 and 64. The DCvoltage provided by the circuit 59 is applied to a hyporesonant typeconverter circuit 60, similar to the above-described converter circuits34 and 44. This converter circuit 60 comprises two MOSFET transistors T₇and T₈ series connected between the output terminals of the circuit 59and a resonant circuit comprising capacitors C₁₁ and C₁₂, coil L₃ andthe primary winding of a pulse type transformer 61. A terminal of thecoil L₃ connected directly to the common point of the transistors T₇ andT₈, while the other terminal is connected to the common point of thecapacitors C₁₁ and C₁₂ through the primary winding of the transformer61.

The secondary winding of the transformer 61 is connected to a rectifyingand filtering circuit 62 which comprises diodes D₉, D₁₀, D₁₁ and D₁₂mounted as a rectifier bridge, a filtering electrolytic capacitor C₁₃and a resistive divider bridge comprising the resistors R₇ and R₈.

This circuit 62 provides a DC voltage E which is regulated andadjustable by means of a circuit 65. This circuit 65 receives a voltagethat mirrors the voltage E provided by the resistive bridge and avoltage E_(C) chosen by the user. This circuit 65 thus gives pulses tocontrol the transistors T₇ and T₈ in such a way that the difference(E-E_(c)) is null, which means that E=E_(C). This circuit 65 may be ofthe voltage/frequency converter type.

This DC voltage E is applied to the converter circuits 34 and 44described here above, and to the switching circuit 53. The switchingcircuit has a first MOSFET transistor T₁ which is supplied with thevoltage E by means of a load resistor R₅ in series with the drain. Thisdrain of the transistor T₁ is connected to a terminal of a first primarywinding 54₁ of the transformer 54 by means of a capacitor C₁₀, the otherterminal of this first winding being directly connected to the positiveterminal of the voltage E.

The switching circuit 53 comprises a second MOSFET transistor T₂ whichis supplied with the voltage E by means of a load resistor R₆ in serieswith the drain. This drain of the transistor T₂ is connected to aterminal of a second primary winding 54'₁ of the transformer 54 by meansof a capacitor C₉, the other terminal of this second winding beingdirectly connected to the positive terminal of the voltage E.

The control electrodes of the transistors T₁ and T₂ are connected to acontrol circuit 66 which gives pulses 70 and 71, respectivelyrepresented by the timing diagrams of FIGS. 5a and 5b. This controlcircuit 66 is itself controlled by a microprocessor 67 which defines thetime intervals between the first pulse 70 and the first pulse 71, thenbetween this first pulse 71 and the second pulse 70, and then betweenthis second pulse 70 and the second pulse 71.

By way of an example, the pulses 70 and 71 have a fixed duration of sometens of microseconds while the time interval between them is of theorder of one millisecond to a few milliseconds.

The two primary windings 54₁ to 54'₁ are wound in reverse directions,and this is also the case for the corresponding secondary windings 54₂and 54'₂. The result of these directions of the windings is that thevoltages V₃ and V'₃ appearing respectively at the output terminals ofthe secondary windings 54₂ and 54'₂ have opposite directions and areequal in terms of absolute value if the primary and secondary windingsare identical.

The circuit 55 for mixing the voltages +V₁, +V₂ and V₃, V'₃ includesdiodes D₁, D₂, D₃ and D₄. The diode D₁ has its anode connected to theoutput terminal 38 (voltage -V₁ ) of the rectifying and filteringcircuit 36 and its cathode connected, firstly, to the anode of the diodeD₂ and, secondly, to an output terminal of the secondary winding 54₂,the other output terminal of said secondary winding 54₂ constituting theoutput terminal 56 of the circuit 55 invention. The cathode of the diodeD₂ is connected to the output terminal 47 (voltage +V₂) of therectifying and filtering circuit 46. The diode D₃ has its anodeconnected to the output terminal 48 (voltage -V₂) of the rectifying andfiltering circuit 46 and its cathode connected, firstly, to the anode ofthe diode D₄ and, secondly, to an output terminal of the secondarywinding 54'₂, the other output terminal of said winding constituting theoutput terminal 57 of the circuit 55. The cathode of the diode D₄ isconnected to the output terminal 37 (voltage +V₁) of the rectifying andfiltering circuit 36. Furthermore, the third output terminal 58 of themixer circuit 55 is connected, firstly, to the common point of theresistors R₁ and R₂ of the rectifying and filtering circuit 36 and,secondly, to the common point of the resistors R₃ and R₄ of therectifying and filtering circuit 46.

The voltage VS₁, which is applied between the filament and the electrodeW₁, is taken between the output terminals 58 and 56 while the voltageVS₂, which is applied between the filament and the electrode W₂, istaken between the output terminals 58 and 57. The capacitors C₁₄ and C₁₅represent the parasitic capacitances of the conductors whichrespectively connect the output terminals 56, 57 and 58 to the electrodeW₁, the electrode W₂ and the filament 12.

The operation of the device of FIG. 4 shall now be explained withreference to the graphs of FIGS. 5a, 5b, 5c and 5d. FIGS. 5a and 5brepresent the timing diagrams of two consecutive pulses 70 and 71 whichdefine the instants at which the voltages applied to the electrodes W₁and W₂ are switched. Naturally, in each diagram, these pulses arerepetitive and have a duration of about twenty microseconds. The pulse70 and the following ones (not shown) are applied to the controlelectrode of the transistor T₁ while the pulse 71 and the following ones(not shown) are applied to the control electrode of the transistor T₂.

The pulse 70 saturates the transistor T₁ in such a way that a positivevoltage +V₃ appears at the terminals of the secondary winding 54₂ and anegative voltage -V₃ appears at the terminals of the secondary winding54'₂. Since the absolute value of this voltage V₃ is greater than theabsolute values of the voltages V₁ and V₂, the capacitors C₁₅ and C₁₄are respectively charged at the voltages:

    VS.sub.1 =+V.sub.3 -V.sub.1 (FIG. 5-c)

    VS.sub.2 =-V.sub.3 +V.sub.2 (FIG. 5-d)

Indeed, a current for the charging of the capacitor C₁₅ flows from theoutput terminal 56 to the terminal 58, and then to the terminal 29, inthe capacitor C₁ towards the terminal 38 and returns to the other outputterminal of the secondary winding 54₂ by the diode D₁ which isconductive while the diode D₂ is off.

Besides, a current for the charging of the capacitor C₁₄ flows from theoutput terminal 57 to the terminal 58, and then to the terminal 49, inthe capacitor C₃ towards the terminal 48 and returns to the other outputterminal of the secondary winding 54'₂ by the diode D₃ which isconductive while the diode D₄ is off. The voltage VS₁ remains at thevalue (V₃ -V₁) after the end of the pulse 70 owing to the presence ofthe voltage +V₂ which keeps the diode D₂ in an off state. In the sameway, the voltage VS₂ remains at the value (-V₃ +V₂) after the end of thepulse 70 owing to the presence of the voltage +V₁ which keeps the diodeD₄ off.

The pulse 71 saturates the transistor T₂ in such a way that a negativevoltage -V₃ appears at the terminals of the secondary winding 54₂ and apositive voltage +V₃ appears at the terminals of the secondary winding54'₂, namely biases opposite to those resulting from the pulse 70, thesebiases being indicated by the corresponding arrows. The capacitors C₁₅and C₁₄ are respectively charged at the voltages:

    VS'.sub.1 =-V.sub.3 +V.sub.1 (FIG. 5-c)

    VS'.sub.2 =+V.sub.3 -V.sub.1 (FIG. 5-d)

Indeed, a current flows in the capacitor C₁₅ from the terminal 58 to theterminal 56, in the winding 54₂, in the conductive diode D₂ (the diodeD₁ is off) towards the terminal 47, in the capacitor C₄ towards theterminal 49 which is connected to the terminal 58.

Besides, a current flows in the capacitor C₁₄ from the terminal 58 tothe terminal 57, in the winding 54'₂, in the conductive diode D₄ (thediode D₃ is off) towards the terminal 37, in the capacitor C₂ towardsthe terminal 39 which is connected to the terminal 58.

The voltage VS'₁ remains at the value (-V₃ +V₂) after the end of thepulse 71 owing to the presence of the voltage -V₁ which keeps the diodeD₁ in an off state. In the same way, the voltage VS'₂ remains at thevalue (V₃ -V₁) after the end of the pulse 71 owing to the presence ofthe voltage -V₂ which keeps the diode D₃ in its off state.

Should the time interval between the switch-over pulses 70 and 71 begreat, and should this lead to a slight discharge of the capacitors C₁₅and C₁₄, they can be kept at the charging voltage through the renewedapplication of one or more pulses 70 for the time interval between apulse 70 and a pulse 71 or one or more pulses 71 for the time intervalbetween a pulse 71 and a pulse 70. Thus, the voltages VS₁, VS₂ and VS'₁,VS'₂ may be kept at these values for as long as is desirable, and evencontinuously.

To modify the voltages VS₁, VS₂ and VS'₁, VS'₂, it is sufficient tomodify ±V₁ and ±V₂ in changing the corresponding frequencies F₁ and F₂by means of the microprocessors 31 and 41 respectively.

Since all the voltages ±V₁, ±V₂ and ±V₃ are obtained at the transformersecondary winding terminals 35, 45 and 54, the output terminals 56, 57and 58 may be carried to any potential, for example that of the cathode,namely -75 kilovolts, without using any special protection circuit.

What is claimed is:
 1. A device used for providing switched X-ray tubeelectrode biasing voltages VS₁, VS₂ or VS'₁, VS'₂, comprising:firstmeans for generating a first pair of adjustable DC voltages +V₁, -V₁with equal and opposite amplitudes, second means for generating a secondpair of adjustable DC voltages +V₂, -V₂ with equal and oppositeamplitudes, third means for generating a pair of pulse voltages +V₃, -V₃with equal and opposite amplitudes, and fourth means connected to saidfirst, second and third means, for combining the pair of pulse voltages+V₃, -V₃, at determined instants, with one of the voltages of the firstand second pairs of DC voltages so as to charge capacitors and obtain DCvoltages

    VS.sub.1 =V.sub.3 -V.sub.1 and VS.sub.2 =-V.sub.3 +V.sub.2

during a certain period of time, and provide DC voltages

    VS'.sub.1 =-V.sub.3 +V.sub.2 and VS'.sub.2 =+V.sub.3 -V.sub.1

during another period of time.
 2. A device according to claim 1, whereinsaid third means comprise a pulse type transformer comprising twoprimary windings and two secondary windings in opposite directions,which are powered by a DC voltage E that is constant and regulated bymeans of two switches, each switch being normally open and being closedby a pulse.
 3. A device according to claim 2, wherein said fourth meanscomprise:a first pair of diodes in series, the common point of which isconnected to an output terminal of one of said two secondary windings,the anode of one of the diodes being connected to the terminal of saidfirst means generating the voltage -V₁ and the cathode of the otherdiode being connected to the terminal of said second means generatingthe voltage +V₂, and a second pair of diodes in series, the common pointof which is connected to an output terminal of said other secondarywinding, the anode of one of the diodes being connected to the terminalof said second means generating, the voltage -V₂ and the cathode of theother diode being connected to the terminal of said means generating thevoltage +V₁.
 4. A device according to any of claims 1, 2 or 3, whereinsaid first means or second means each comprise:supply means forsupplying a constant DC voltage E, means for converting said DC voltageE so as to obtain AC pulses with a frequency F, each corresponding to aquantity of electricity that is constant from one pulse to the next one;means for rectifying and filtering said AC pulses so as to obtain saidDC voltage V_(p), means for modifying the frequency F of said AC pulsesas a function of the DC voltage V_(p) that is to be obtained.
 5. Adevice according to claim 4, wherein said means for converting saidconstant DC voltage E comprise a resonant circuit, the resonancefrequency of which is greater than the frequency F.
 6. A deviceaccording to claim 4, wherein said means for modifying the frequency Fof said AC pulses comprise:means for determining, by calibration, thefrequency F of said pulses as a function of the voltage V_(p) to beobtained, means for generating control pulses at the frequency F fromthe information on the value of said frequency F, said pulses beingapplied to said means for converting said DC voltage E.
 7. A deviceaccording to claim 6, wherein said means for generating control pulsesat the frequency F comprise:a counter circuit that provides the Ffrequency pulses, and a logic circuit that provides signals forcontrolling said converter means for converting the voltage E, theduration of said signals being greater than one half-period but smallerthan said resonance period, the repetition period of said signals beingat most equal to said resonance period.
 8. A device according to claim7, wherein said logic circuit comprises:a first AND circuit, one of thetwo input terminals of which is connected to the output terminal, ofsaid counter circuit, a bistable circuit, the control input terminal ofwhich is connected to the output terminal of said first AND circuit soas to change state at each signal produced by the first AND circuit; asecond AND circuit, one of the two input terminals of which is connectedto the output terminal of said bistable circuit corresponding to thestate 1; a third AND circuit, one of the two input terminals of which isconnected to the output terminal of said bistable circuit correspondingto the state 0; a first delay circuit, the input terminal of which isconnected to the output terminal of said first AND circuit and theoutput terminal of which is connected to the second input terminal ofsaid first AND circuit, and a second delay circuit, the input terminalof which is connected to the output terminal of said first AND circuitand the output terminal of which is connected to the other inputterminal of said second and third AND circuits.